How can the degree of polymerization / molecular weight of a chitin/chitosan be determined?
There are different ways to determine the degree of polymerization / molecular weight of chitins and chitosans. In the case of chitin oligomers and chitosan oligomers, the most precise method is mass spectrometry (MS) that measures the mass of ionized molecules which can then easily be used to calculate the degree of polymerization and the degree of acetylation of the sample. Importantly, mass spectrometry will not give an average value for DP and DA but will separate the oligomers according to their molecular weight so that the composition of a sample can be determined with accuracy (at least in terms of degrees of polymerization and acetylation, while the pattern of acetylation is more difficult to determine). This is not the case for chitin and chitosan polymers where the molecular weight determined will always constitute an average value, where the polydispersity index informs about the distribution of molecules in the sample. The most frequently used methods to determine molecular weights of chitosans are viscosimetry and size exclusion chromatography (SEC). The former does not require expensive infrastructure and is simpler to perform, but only the latter can inform about the polydispersity index also, and can also give absolute values for the molecular weight if an appropriate (MALLS) detector is used. SEC-MALLS is considered the gold standard for MW/DP analysis. In order to calculate the average degree of polymerization from the average molecular weight determined, knowledge of the average degree of acetylation of the sample is required in any case.